This invention relates generally to the field of directing antennas, and more particularly, to directing an antenna to receive digital television signals.
FIG. 1 shows a distribution of energy versus frequency for a conventional television (TV) signal 100, for example, NTSC, PAL, or SECAM. The signal 100 includes three energy peaks, one for video 110, one for color 120, and one for sound 130. As can be seen, conventional television transmitters concentrate most of the energy of the radio frequency (RF) signal in a relatively narrow bandwidth near the frequency of the picture sub-carrier, i.e., xcx9c1 MHz. Therefore, an antenna designed to receive conventional (terrestrial-based analog) TV signals can usually be directed for optimal reception of the video portion by only considering the strength of the signal.
FIG. 2 shows a distribution of energy versus frequency for an advanced television (ATV) signal 200. An advanced television signal can concurrently carry a variety of multimedia content, for example, HDTV, conventional TV, video-text, audio, low-bandwidth TV, etc. In the ATV signal 200, the energy of the signal, at the transmitter, is distributed substantially uniformly over the entire channel bandwidth, usually 6 MHz. With such a wide spectrum signal, the probability of destructive ghost interference is significantly higher than in the case of conventional TV that has a narrow spectrum signal. As a result, static and dynamic multi-path fading are more likely to corrupt the spectrum of the received ATV signal than in the case of the conventional TV signal. This interference is shown by xe2x80x9cnotchesxe2x80x9d 201-202 in FIG. 2.
Multi-path fading is a result of mostly two effects. The first effect is caused by variations in the index of refraction due to spatial and temporal variations in temperature, pressure, humidity, and turbulence in the atmosphere. These varying atmospheric conditions result in multiple paths from the transmitter to the receiver, each path having a different effective electrical length. The second effect is due to the reflection of the RF signal from different obstacles or objects in the signal path. The second effect produces a more stable multi-path environment when the obstacles or objects are stationary. In either case, the signals arriving at the antenna via different length electrical paths interfere with each other.
It is possible to describe the effect of multipath fading on a passband signal as a superposition of a number of electromagnetic waves. For an ATV terrestrial signal, the highest passband frequency is, for example, 6 MHz. The delay along multiple paths can be in the range of xe2x88x922 to +25 xcexcs.
The notches 201-202 in the power spectrum will happen when several components of the signal approach the receiver at the same passband frequency but different phases. The depth of a notch can be equal to the full power when the two paths are nearly the same amplitude but opposite phase. In this case, destructive interference results in zero energy at this point in the power spectrum. The ATV receiver cannot process the signal and the receiver effectively becomes inoperative.
Anecdotal evidence has digital television receivers from different manufacturers standing side-by-side in a retail store, each hooked-up to the same antenna, some working perfectly, others totally inoperative. Attempts to xe2x80x9ctunexe2x80x9d the sets based on built-in signal strength meters frequently are futile or give inconsistent and unpredictable results.
Consequently, in order to determine the optimum direction of a receiving antenna for an ATV receiver, the strength of the received signal alone is not enough to determine the optimal antenna direction. Therefore, it is desired to provide a method and apparatus which can direct an antenna to optimally receive advanced television signals.
Provided is a method and apparatus for measuring the strength and quality of a digital television signal. The measured values can be used to optimally direct an antenna to an orientation which maximizes the quality of the signal.
Specifically, the invention measures the strength of the signal as a function of the azimuth angle of the antenna. This can be done in the tuner section of a television receiver using an automatic gain control circuit. The flatness of the signal, as a function of the azimuth angle of the antenna, is measured in an adaptive equalizer of the receiver.
These two measured values can be displayed on the screen of the receiver, and the antenna can be adjusted to maximize the flatness of the signal while maintaining the strength of the signal above a minimum threshold. Alternatively, the antenna can be automatically adjusted.